Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other objects) as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a toner image. Note that the toner image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., “fuse”) the print image to the receiver. Plural print images (e.g., separation images of different colors) can be overlaid on the receiver before fusing to form a multicolor print image on the receiver.
Typically, a linear printhead including an array of LED light sources is used to form the electrostatic latent image. Differences between the power provided by the individual light sources can result in streak artifacts being formed in the printed image. Even if the printhead is carefully calibrated in the factory to equalize the power provided by each light source, it has been found that when the printhead is installed into a printing system that there can be residual streak artifacts, and that these artifacts can change with time. Therefore, there remains a need for a method that can be performed in the field to calibrate the printhead to equalize the exposure provided by each light source.
Typically, a linear printhead including an array of LED light sources is used to form the electrostatic latent image. The printhead generally has an 8-bit interface which enables 256 different exposure levels to be provided by each of the light sources. The exposure level provided by the light sources is typically controlled by adjusting a time that the light sources are activated, where each of the pixel code values is mapped to an exposure time that provides an aim exposure level.
To control the exposure time, some printheads utilize a stream of exposure clock pulses having non-uniform pulse widths, where the pulse widths are selected to provide the aim exposure levels. The exposure time for a particular pixel is controlled by counting a number of exposure clock pulses corresponding to the pixel code value. For example, for a pixel code value of 100, the light source would be activated for 100 exposure clock pulses. However, it has been found that the optical power provided by the light sources is not constant with time, and that the shape of the light output function is a function of the pulse widths of the exposure clock pulses. Therefore, determining the pulses widths required to provide the desired aim exposures can be a complex process because changing the pulse width to modify the exposure time changes the power, which will in turn affect the exposure time required to provide the aim exposure. In some printing systems, the aim exposure level as a function of pixel code value may be updated in the field as part of a printer calibration process. It is therefore necessary to update the pulse widths of the exposure clock pulses accordingly. There remains a need for an efficient method for determining a pulse timing function that can be implemented in the field, and for controlling the printer with a pulse timing function appropriate for a particular print mode.